1. Field of the Invention
The present invention relates to a sheet processing apparatus and an image forming system.
2. Description of the Related Art
There are widely known sheet processing apparatuses disposed on the downstream side of a main body of an image forming apparatus and performing post-processing such as binding recording sheets or the like output from the image forming apparatus. Nowadays, such sheet processing apparatuses have been multifunctionalized, and generally perform not only conventional edge binding but also saddle-stitching and bookbinding. To further improve output quality in the saddle-stitching and bookbinding, there are sheet processing apparatuses that cut the edge of a bound booklet after the bookbinding using a cutter.
In the cutting process of such a cutter, the booklet is conveyed by a conveying unit such as a belt, and positioned by abutting the booklet on abutting stoppers placed in accordance with the size and cutting amount of the booklet. Thereafter, the booklet is fixed by being pressed by a pressing unit, and cut at the edge thereof by a cutting unit. By this operation, the bound booklet is trimmed at the edge thereof.
However, in conventional cutters, because the conveying unit such as a belt is rotating in the state in which the booklet is abutted on the positioning stopper, deflection of a surface sheet occurs. Fixing and cutting the booklet in this state can lead to misalignment at the edge after the cutting process. In addition, during the processes of flatting bulges of the booklet and applying pressure to the booklet, the surface or the spine side of the booklet may be deflected, and moreover, the booklet may move back and forth and be pressed in a displaced state. If the cutting process is performed in this state, misalignment occurs at the edge of the booklet after the process.
For that reason, for example, an invention disclosed in Japanese Patent Application Laid-open No. 2004-196494 is known as a device that performs cutting so as not to produce such deflection. In this invention, after sheets are folded and before the sheets are fixed by being pressed by a bundle pressing member, a plate, a roller, or the like gradually applies pressing force from the spine side (for example, an edge on the side on which the folding process has been performed) of the sheets while moving toward an edge surface so that no deflection is produced between the pressed position and the spine side of the sheets.
However, although the invention disclosed in Japanese Patent Application Laid-open No. 2004-196494 enables straightening of the deflection of the booklet, the booklet may move in the edge surface direction when the pressing force is applied for holding the booklet. If the booklet moves in the edge surface direction and is cut in that state, the booklet is trimmed with a cut surface thereof inclined. If the booklet is trimmed in such a manner, cutting accuracy cannot be improved even if the deflection is straightened.
Therefore, there is a need to suppress such deflection and to prevent misalignment from occurring when a booklet is pressed, whereby accurate sheet processing can be performed.